Concentrations And Deposition Fluxes Of Heavy Metals In Precipitation In Core Urban Areas,Chongqing

Heavy metals with high toxicity and persistence have a great harm to human health. They can be introduced into body through food chains, and accumulate in the body for a long time. Heavy metals are widely distributed in the environment, mainly in the particulate form in the atmosphere. They can enter body through respiratory system, and further damage to human health. In addition, they also can be deposited to water terrestrial ecosystem surface via dry and wet deposition, leading to persistent negative impacts to the ecosystem. It was previously reported that heavy metals with biotoxicity in the atmosphere could be eliminated by wet deposition. Thus, conducting the study of distribution of heavy metals in precipitation and their deposition fluxes has a great significance.Wet deposition studies of heavy metals have been conducted early oversea and some long-term complete monitoring systems have been formed, such as AMAP, EMEP, Helsinki Commission and OSPAR Commission. Even though heavy metals in precipitation have been reported in China, long-term and systematic monitoring systems have not been established.Chongqing, locates in southwestern China, is regarded as a traditional industrial city. With the rapid economy development in recent10years, energy consumption has increased significantly. In addition, Chongqing surrounded with mountains and hills, almost no wind or breeze all year, resulting in high atmospheric stability and difficult diffusion for heavy metal pollutants, and further easily leading to heavy metal pollution in local atmosphere. Therefore, the long-term monitoring of concentrations and deposition fluxes of heavy metals has a great significance to the study of heavy metals sources and their impacts on urban ecosystem. In this study, three sampling sites in core urban area in Chongqing were selected according to the geographic distribution and the extent of effects of human activities. These three representative sites included Nanan (NA) site located in the center of core urban areas, Beibei (BB) site located in suburbs and Jinyun (JY) site located in natural conservation area. Precipitation samples at three sampling sites were collected using automatic samplers from December2011to November2012, mercury and13other heavy metals in precipitation were systematically measured. The levels and deposition fluxes of heavy metals were also analyzed. The results are as follows:(1) During the study period (December2011-November2012), the average rainfall was797.4mm, and pH values of precipitation ranged from2.95to7.70with high frequency acid rain. The electrical conductivity values ranged from6.9to509μS with rainfall as an important factor for their variations; Main ions composition in precipitation was very similar between JY and BB sites with Cl-accounting for the highest proportion followed by SO42-which accounted for the largest proportion at NA site (27.66%).(2) The concentrations of most heavy metals in precipitation were high in precipitation in Chongqing. The volume weighted average (VWM) concentrations of total mercury (THg) and methyl mercury (MeHg) were38.25ng·L-1and0.58ng·L-1, respectively, and the VWM concentrations of Pb and Cd were37.94μg·L-1and0.55μg·L-1, respectively, significantly higher than those reported in North America and the environmental background values in our country. In addition, obvious seasonal variations of heavy metals concentrations in precipitation were found with larger levels in autumn and winter and smaller values in spring and summer. However, spatial variations of heavy metal concentration were not obvious. Correlation analysis showed that rainfall negatively correlated with the concentrations of heavy metals.(3) Significant differences were discovered among the deposition fluxes of heavy metals in precipitation. The Zn and Fe have larger deposition fluxes which were76.26mg·(m2·a)-1and73.03mg·(m2·a)-1and Hg deposition flux (30.50μg·(m2·a)-1) was the smallest. Compared with other researches in the word, the deposition fluxes of heavy metals in precipitation in Chongqing are higher. Obvious seasonal variations were found among the deposition fluxes of heavy metals with larger values in spring and summer and smaller in autumn and winter, oppositing to seasonal variations of heavy metal concentrations. However, spatial variations of heavy metals deposition were not obvious.(4) By introducing enrichment factor (EF), EF of Fe, Mn, V and Co were less than10and other most elements in our study are greater than10. EF of Cu, Pb, Zn, Cd, Ag, As, Se and Hg were greater than100, and EF of Cd and Se were even up to1740and4133, respectively, suggesting that these two metals were influenced seriously by human activities.(5) Principal component analysis showed that the deposition fluxes of heavy metals in precipitation in Chongqing may be influenced by coal burning, vehicle exhaust emissions, landfill and incineration. Moreover, HYSPLIT4analysis showed the airflow arrived in Chongqing through some contaminated areas, and thus these airflows may also be another important source leading to high concentrations of heavy metals in precipitation in Chongqing.In addition, further research can focus on the dry deposition in the atmosphere, by selecting appropriate models or accurate determination methods to estimate dry deposition fluxes, with the aim to have a better understanding of the global biogeochemical cycle of heavy metals.